U.S. Attorney Joseph E. diGenova has denied charges that he was a source of news reports about a federal grand jury investigating alleged drug use by District government employes and said there is no political motivation behind the probe.

D.C. Mayor Marion Barry, who testified before the grand jury in January, has accused Reagan appointee diGenova and his office of politically motivated "leaks" to embarrass the Barry administration and indirectly the presidential campaign of Democrat Walter F. Mondale.

"Ongoing investigations are never discussed, and they should not be. And they are not," diGenova said during a wide-ranging interview this weekend with Larry King on Mutual Broadcasting System radio (WTOP). "That's been the policy . . . of my predecessors, and it's been my policy.

"I have not, privately or publicly, discussed any investigations, because that simply serves no useful purpose."

One of the ground rules of the nationally broadcast interview was that diGenova would not talk specifically about any investigation, including the drug probe.

But King, in asking about leaks, noted news reports that the grand jury is looking into the possibility of perjury by Barry in his January testimony.

A New York Times article about Barry appeared as he and other black leaders were meeting in Minnesota with Mondale, prompting Barry's charge that the reports were timed to embarrass the presidential campaign.

Barry testified in January about Karen K. Johnson, a former city employe and friend of Barry's who has since been convicted of selling cocaine. The mayor has repeatedly denied using illegal drugs or knowing of Johnson's involvement with drugs.

He did acknowledge through his legal counsel, Herbert O. Reid Sr., having a "personal relationship" with Johnson and visiting her apartment "on and off" for 12 to 18 months ending in the fall of 1982.

Until recently, Barry refused to talk about the drug probe, saying it would be improper for him to talk about an ongoing investigation, but in past weeks he has taken a new offensive aimed at the U.S. attorney's office and diGenova.

Barry has likened the probe to past lynchings of blacks and "witch hunts" of the McCarthy era and has charged that diGenova or his office are the source of the reports.

Reid on Friday wrote to U.S. Attorney General William French Smith suggesting that a special prosecutor be appointed to determine if diGenova or others in his office have been leaking information.

Smith earlier had declined to discuss the D.C. drug probe or Barry's allegations but told reporters he is "as distressed by leaks as anybody else is."

Justice Department spokesman John Russell said the department would have no comment on Reid's letter or suggestion.

DiGenova, in the radio broadcast, said that "99 percent of the time" information about grand jury investigations comes from the witnesses. Secrecy rules bar federal investigators, prosecutors and grand jurors from revealing information, but it is legal for witnesses to talk about their testimony and any information they are given during the proceedings, he said.

"And that frequently is the way information gets out. Although it is termed a 'leak,' it is not in fact a leak, because the individuals who might choose to give it out are not bound by any secrecy," diGenova said.

He called leaks of information "extremely annoying," saying that the key to a successful investigation is secrecy.

"There are no politics involved in any investigations," diGenova said. "Prosecutors receive either credible or incredible allegations, specific allegations of crime. If the allegations are credible, and they are reasonable, it is the responsibility of a prosecutor to investigate them."

The career federal prosecutor system insulates prosecutors from being "used" for political aims, he said.